Seismometers used to track whales

Last updated 07:55 14/05/2013

LOUD BEAST: The fin whale.

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At first, the whale chatter was just a nuisance.

When Seattle scientists set out to monitor earthquakes off the Northwest coast, they expected their underwater seismometers to occasionally pick up the booming voice of the fin whale - the second-largest creature on Earth. But what they wound up with was such a cacophony that they had trouble zeroing in on the actual tremors.

"It was a very big pain," said William Wilcock, a marine geophysicist at the University of Washington.

To separate tens of thousands of small quakes from hundreds of thousands of whale calls, he and his students were forced to write their own automated program. Now, they're mining that mountain of discarded data for insights into one of the sea's least-known and most endangered giants.

"Basically, we can track the whales using techniques very similar to the ones we use to locate earthquakes," Wilcock said.

Second only to blue whales in size, fin whales remain shrouded in mystery because they're so challenging to study. Nobody knows where they give birth or even why they sing.

"When I started this project, I was really surprised by how little is actually known about them," said doctoral student Michelle Weirathmueller. With seafloor seismic monitoring on the rise, her goal is to fill in some of those gaps.

"The thing that's neat about this is that we are basically piggybacking on experiments that were designed for something completely different," she said.

Already, the UW team has discovered that some fin whales migrate north in the fall - a time when biologists assumed most of the whales would be headed south to breed.

Learning more about the species' movements, behaviour and communication patterns could bolster efforts to protect fin whales from further harm, now that they are beginning to rebound from decades of whaling, said John Calambokidis, whale biologist and co-founder of the Olympia-based Cascadia Research Collective.

A recent study found that fin and blue whales are more likely to be struck by ships than any other whale species on the West Coast, he pointed out.

Puget Sound residents got a gory demonstration last month, when part of a fin-whale carcass washed up on a beach near Burien. Scientists said the animal was killed by a ship and dragged into the sound.

Usually, fin whales stick to deep water offshore _ which is one of the reasons they haven't been studied much, Calambokidis said. Tagging and tracking is expensive, and so are ship-based expeditions to observe and follow whales.

Fixed arrays of seismic instruments offer a promising alternative.

"It's clearly going to be an important and very-cost effective future direction," he said.

Reaching up to 85 feet (26 metres) in length and weighing in at up to 80 tons, fin whales earned the nickname "greyhounds of the sea" because of their sleek build and explosive speed. They also produce some of the loudest noises in the animal kingdom.

"It's just this amazing sound," Weirathmueller said.

The whales let loose with their short, basso profondo blasts roughly every 25 seconds, often continuing for a day or more. They pause only to breathe. The pattern is so regular that scientists who first recorded the song thought it was coming from some man-made device.

The whales' calls can travel thousands of miles through water. If unleashed on land, the average decibel level would be deafening to a human. But the sound is so deep - deeper than thunder or the biggest bass drum - that only the sharpest human ears could detect it.

In order to listen to the sounds in the lab, Weirathmueller and her colleagues have to speed up the recordings.

That low-frequency, rumbling quality is why the calls of fin and blue whales register on underwater seismometers, which are tuned to detect the rumblings of the Earth. Scientists have used the instruments to track whales before, but never in such large numbers or over such an extended period of time as in the UW studies.

Doctoral student Dax Soule analysed a year's worth of data - more than 300,000 whale calls - recorded by eight seismometers on the ocean bottom off the coast of Vancouver Island.

Wilcock and his colleagues installed the instruments beginning in 2003 to record geologic activity at the Juan de Fuca Ridge, where molten rock rising from the Earth's interior creates new seafloor and fuels underwater volcanoes and hydrothermal vents.

In the winter, the number of whale calls was stunning.

"There are days when it's really like whale soup out there," Soule said.

Using a computer program of his own design, Soule was able to tease out the paths followed by 154 individual whales or groups as they passed through the area.

The conventional wisdom holds that only males sing, with the goal of attracting females - but even that isn't known for sure, Calambokidis said.

Soule's data revealed two distinct types of calls, one slightly higher pitched than the other. Most of the animals he tracked were moving south in the winter and early spring, as expected. Some seemed to travel solo, while others moved in pairs or clusters. Why some animals opted to head in the opposite direction is an open question, Soule said.

One suggestion is that they might be bachelor males, with no incentive to visit the breeding grounds. But that's just guess, he cautioned.

Seismometers only detect the loudmouths, Calambokidis pointed out, which is one of the technique's shortcomings. That's why it would be ideal to mount a research blitz, where the signals from the seafloor instruments could be correlated with observations of the whales' behaviour and even some tagging, he said.

Similar studies of blue whales revealed that most of the animals were silent, with just a small number of voluble males making all the racket.

Weirathmueller is expanding the UW studies by tapping into new seismic networks.

As part of a project called the Cascadia Initiative, scientists recently deployed 70 seismometers off the Northwest coast, where they've been gathering earthquake data for more than a year. An underwater observatory called NEPTUNE Canada includes seismometers, and a similar array of instruments will be installed soon off the Washington coast.

The additional sensors will allow Weirathmueller to follow the whales over a larger area and identify where they congregate and feed.

"If we want to prevent things like ship strikes and entanglements," she said, "the most important thing is to know where the whales are."